Apparatus of the foregoing kind may be referred to as flow cytometric sorting systems and are used in the medical research and diagnostic field for the rapid analysis of blood cells and other biological cells. Systems for cell separation and sorting are described in U.S. Pat. Nos. 4,038,556; 3,963,606; 3,710,933 and 3,380,584, in SCIENCE, Vol. 198, pages 149-157, published Oct. 14, 1977, and in the references cited therein.
U.S. Pat. No. 4,038,556 discloses a method of and apparatus for the simultaneous optical measurement of several characteristics of each particle of a group of small particles while the particles are suspended in a liquid.
U.S. Pat. No. 3,963,606 discloses a particle separator for separating particles in a liquid according to certain characteristics including a device for adjusting an electrical delay to be equal to the time between the emergence of a particle from a jet forming aperture to the breakoff point.
U.S. Pat. No. 3,710,933 discloses an apparatus for automatically analyzing and sorting minute particles suspended in a liquid on the basis of certain preselected characteristics.
U.S. Pat. No. 3,380,584 discloses a particle separator in which electrical pulses cause an acoustic coupler driver to vibrate the fluid which contains the particles.
The SCIENCE article, Vol. 198, pages 149-157 discloses a flow cytometer. Fluorescence from biological cells within a fluid stream is measured at the intersection with a laser beam. Droplets containing cells of interest are sorted out of the fluid stream. To the extent that it might be necessary to understand fully the techniques involved and the teachings of the invention herein, the above patents are incorporated herein for reference.
A major problem in using the cell sorter systems wherein a jet stream subjected to vibrations breaks off into droplets is ascertaining if a change has occurred in the point at which the jet stream is breaking into droplets. If the precise instant of breakoff with relation to the sense point changes, then the instrument ceases to be a cell sorter and becomes a water or saline solution sorter of unknown cell content. An even worse undesired result is that the unwanted particles are sorted when there is such a change in the breakoff point. Such a change is often due to changes in the mechanical coupling coefficients, such as air bubbles entering the flow chamber and partial plugs of the jet stream exit orifice. Optical sensitivities, such as the presence of undesired light, render impractical the use of an illuminating strobe source to observe the breakoff point on the jet stream while the system is taking data or sorting the cells. It is at this time of sorting that a monitor is most needed.
Furthermore an additional problem exists with respect to the sense diode used in detecting such change in the breakoff point. When the radiation beam which provides information concerning said changes is constructed and arranged so that the radiation beam moves on and off the sense means, and in particular, when the beam is totally on or off it, a saturation effect results and the signal produced by the sense means becomes constant thereby limiting the range of useful measurements.